A female suicide bomber blew herself up at a police station in Istanbul's historic Sultanahmet district on Tuesday, killing one officer and wounding another.
There was no immediate claim of responsibility; but the bombing comes less than a week after far-left group DHKP-C said it was behind a grenade attack on police near the prime minister's office in Istanbul.
Turkey also faces possible threat from Islamist militants moving across the border from Syria and Iraq and, despite a truce in a 30-year-old insurrection, from Kurdish rebels.
Police sealed off the street where Tuesday's attack happened, across the square from the Aya Sofya museum and the Blue Mosque and near the Basilica Cistern, which are among the main sites for millions of visitors to Istanbul each year.
"We were shaken by a very loud blast. There were customers and everyone dropped to the floor," Kaan Koc, who works just across from the station, told broadcaster CNN Turk.
"A police officer came out of the station and fired into the air saying 'disperse, there is a suicide bomber, go inside'. Then we heard gun fire but we weren't sure who was shooting."
Windows were shattered and shutters hung unhinged from the yellow, three-storey tourist police station.
The woman entered the police station saying in English that she had lost her purse and then blew herself up, Istanbul Governor Vasip Sahin told reporters at the scene. Her nationality and identity were unknown.
Prime Minister Ahmet Davutoglu said one of the officers had died. It was not immediately clear whether the bomber had links to any particular group.
Attacks on police
The DHKP-C (Revolutionary People's Liberation Party-Front) had warned of further strikes after last Thursday's attack, in which a man carrying an automatic weapon was detained near the Ottoman-era Dolmabahce Palace.
The group was also behind a suicide bombing at the U.S. Embassy last year and numerous attacks on police stations. Most were in outlying Istanbul suburbs, apparent vendettas against particular police officers.
It said Thursday's attack was over the killing of 15-year-old boy who died last March after nine months in a coma from a head wound sustained during an anti-government protest. It blamed President Tayyip Erdogan for the death.
Turkey faces other security threats
Some of the thousands of foreign fighters who have joined the ranks of Islamic State in Syria and Iraq have entered via Turkey, raising concern that they could return and strike on Turkish soil.